Our long-term goal is to understand, at the cellular level, how specific neural activity patterns are selected from multifunctional networks in the intact animal. This project aims to address the largely unknown but prevalent aspect of neural circuit regulation resulting from the parallel influence of multiple, behaviorally-linked modulator inputs (i.e. co-modulation). A set of behaviorally-linked peptide hormones that co-modulate the gastric mill (chewing) motor circuit in the crab stomatogastric nervous system will be identified, and their separate and collective actions on this circuit will be characterized. This work will motivate future studies in the mammalian CNS, insofar as many of the same principles underlie rhythmic motor pattern generation and modulation for all rhythmic movements (e.g. walking, breathing, chewing) in all animals. Specifically, we aim to determine and characterize (1) the behavioral (feeding) state- specific influence of hormone-conveying hemolymph (crab blood) on the gastric mill circuit, and (2) the relationship between co-circulating peptide hormone actions on this circuit and their individual actions. Similar events are not yet elucidated in any defined neural circuit. Hemolymph will first be collected from unfed and recently fed crabs and applied, separately, to the isolated nervous system to determine its effect on the gastric mill (chewing) motor pattern. In parallel, the identity and concentration of the 5 peptide hormones in the hemolymph that exhibit the largest increase from the unfed to fed state will be determined by mass spectrometric methods. These 5 peptides will then be examined, singly and collectively, for their influence on the gastric mill motor pattern. The cellular mechanisms underlying any changes in the gastric mill motor pattern will be determined and compared using current-, voltage- and dynamic-clamp manipulations. This is one of the few biological systems where such a detailed analysis of an identified microcircuit is possible, and where the consequences of in vivo events can be examined in detail with the same system in vitro. These studies will be foundational for understanding comparable events in the mammalian CNS, where circuits are also co-modulated but where a comparable analysis is not yet feasible, and where such systems can become dysfunctional.